1. Field of the Invention
The present invention relates to a color signal processing circuit, an image pickup apparatus, and a color signal processing method that can be favorably applied to adjusting white balance by adjusting the gain of primary color signals of red (R), green (G), and blue (B) (hereinafter referred to as R, G, B color signals) generated by an image pickup element irradiated with light that is incident from a light source via an optical system.
2. Description of the Related Art
In the past, digital still cameras, digital video cameras and the like have been provided as examples of image pickup apparatuses that automatically carry out white balance processing. Here, the term “white balance processing” refers to processing that changes respective gains of the R, G, B color signals obtained from a camera system so that the signals all become equal when picking up an image of a white subject that is illuminated by a given light source.
By carrying out white balance processing, an image pickup apparatus adjusts white color that look colored due to differences between various light sources such as natural light (i.e., sunlight), light from incandescent bulbs, or light from fluorescent bulbs (for example, when the color temperature of a light source is low, the white color looks reddish and when the color temperature of the light source is high, the white color looks bluish) so as to become achromatic white. Processing that automatically adjusts white balance using a blackbody radiation curve showing changes in color temperature of natural light is called “automatic white balance processing”.
However, it is difficult to precisely and universally express a blackbody radiation curve (expressing changes in color temperature as a curve) using an equation. For this reason in the past, white balance has been adjusted by “pulling” the color temperature of a light source to a target value using a rectangular pull frame or by using a blackbody radiation curve that is substantially linearly-approximated.
An example of a pull frame set by existing white balance processing will now be described with reference to FIG. 1. FIG. 1 is a diagram showing one example of a pull frame and a linear approximation of a blackbody radiation curve in a color coordinate system where the vertical axis represents B/G and the horizontal axis represents R/G. The pull frame 101 is an octagon composed of the eight approximate points f1 to f8. A blackbody radiation curve 102 is a broken line that is linearly approximated using approximate points k1 to k4. The blackbody radiation curve 102 is positioned inside the pull frame 101 and the slope of the pull frame 101 is set in accordance with the slope of the blackbody radiation curve 102.
According to the existing method, when automatically adjusting white balance, the color temperature of the surrounding environment (i.e., light source) is obtained to find the present point 103 expressing the position of the light source in a color coordinate system. When the present point 103 is positioned inside the pull frame 101, the present point 103 is pulled toward a convergence target point 104 set on the blackbody radiation curve 102. By pulling the present point 103 in this way, the white balance is adjusted.
Japanese Unexamined Patent Application Publication No. H11-331854 Patent Document 1 discloses a camera system that adjusts white balance, when integrated value data obtained by integrating a color difference signal in each field is present inside a pull limit frame set based on a reference point, by moving the reference point along a blackbody radiation curve in a direction that is asymmetric about the origin of a color coordinate system.